I once had a film cameraman tell me that sync was not important only to get a call later to fix his production. That was many years ago when audio was recorded on a medium called magnetic tape or film. Remember? Now it’s all digital. I really thought that digital audio would resolve issues but now there are a few more details to consider.
Digital audio is recording in slices or audio samples regular time intervals. This is called the sampling rate. The standard for recording digital audio for picture is 48 thousand times per second or 48K. The 48K rate is used because it is mathematically compatible to picture formats. If there is a variance in the audio sampling rate then the sound can drift in relation to the picture and create chaos in the editing room. Even minor drifts can create problems. Basically there will be more or too little audio for a given scene. I am pretty sensitive to this and I see one or two TV commercials weekly with bad lip sync.
- The recording was made at the wrong sample rate, not at 48K sample rate.
- Multiple cameras and audio recorders and no external master sync source.
- No time code is used.
- Delay induced by a digital console without external sync.
- A mistake in the editing room.
- An unexpected equipment failure.
We all know things go wrong so some proper planning and redundancy may save the day. Always test your setup before recording the final product. Digital recording does not mean good recording. I have had to fix many digital recordings that were noisy or distorted. After you do a test recording make sure your location sound man listens to what is being recorded in headphones and listens to the recording of each scene after a take.
If problems do arise, the drift may be resolved by adjusting the playback to match the picture in an audio post house or editing room. Some productions require frame accurate recording and this adjustment is not acceptable if there is budget to re-shoot the scenes. Happy shooting and may the sync be with you.